The invention relates to a filter arrangement, in particular, an arrangement of at least one HF filter that can be used as an input or output filter in the front end of a wireless communication device.
Various types of filters and filter arrangements based on different physical principles are known. One of these is filters that operate with bulk acoustic wave resonators. Because of their small space requirements, the possibility of manufacturing them on an integrated basis and the filter properties that can be achieved with them, bulk acoustic wave resonators for mobile applications are usually constructed as FBAR filters. An FBAR filter comprises at least two flat bulk acoustic wave resonators, which are arranged with an electrode flatly on a substrate. Several resonators can be arranged one above another, but preferably side by side, and connected to one another in an integrated manner via structuring. A functional filter is obtained by connecting several FBAR resonators, with ladder-type and lattice-type arrangements being known.
The electrical contacting of such components and the integration of additional circuit components is preferably done by mounting the substrate, e.g., a silicon substrate, carrying the FBAR resonators in a flip-chip arrangement on the upper side of a carrier, wherein the external contacts of the finished component are realized on the underside of the carrier. These can be connected via through-hole contacts to the contact surfaces present on the surface. For a multilayer carrier, which comprises two or more dielectric layers between which metallization planes are arranged, one can integrate additional passive circuit components in the carrier by structuring the metallization planes.
For protection against environmental influences, particularly moisture and mechanical damage, the component, without an additional housing, can be provided with a covering produced by laminating a protective film that lies tightly against the back side of the carrier and on the upper side of the carrier. A particularly hermetically sealing covering is obtained if a metal film is applied to the upper side of the film covering.
The disadvantageous aspect of such hermetic coverings coated with a metal film is the small distance of this covering from the flatly formed electrodes of FBAR resonators or bulk acoustic wave resonators in general. Considerable parasitic capacitances, which adversely affect the component, can thereby be formed between the electrodes and the metal film of the covering by intervening covering films and the chip substrate, which act as a dielectric. Because of this capacitive coupling, different FBAR resonators are capacitively coupled to one another, which further degrades the filter properties.
Filter arrangements also exist in which two filters are realized on one substrate or on separated ones and also arranged on a common substrate and covered with a common covering. In such an arrangement, there can be a capacitive coupling of the two otherwise separated filters, i.e., so-called crosstalk appears. If the two filters form a duplexer, then the isolation between the transmit path (TX path) and the receive path (RX path) is impaired and reduced by the capacitive coupling and the crosstalk caused thereby.
An increased scattering of high-frequency transmit signal power into the receive path leads to an increase in the noise level, which implies a loss of dynamic range in the preamp stage, even to the point of saturation. This can be compensated only to a limited extent by lower-noise preamps (LNA) with higher linearity (IIP3 point), which are expensive, however, and display other technical disadvantages, such as increased power consumption. Beyond a certain increase in the noise level, the interference is no longer acceptable, if the sensitivity of the receiver no longer suffices even for reception of nearby base stations.
For CDMA mobile telephone systems, such scattering also leads to higher requirements for the suppression of intermodulation products in the digital baseband filtering in the receive path. These intermodulation products arise by conversion of external interference signals at the nonlinear power amplification stage in the transmit path, and lie in the so-called fly-back region at a multiple of the transmission frequency.
For the solution of this problem, it was already proposed to connect the metal film of the covering to a ground terminal and thereby reduce the capacitive coupling. The starting point here was the assumption that as many shunt connections to ground as possible improve the effect.
It was shown, however, that the component properties are not substantially improved in this manner, and that the capacitive coupling of different bulk acoustic wave resonators in a filter arrangement still appears and degrades filter performance.